Pellicle and process for preparing same



Patented Aug. 29, 1939 UNITED STATES PATENT OFFICE Frederick M. Meigs, Wilmington, Del., assignor,

by mesne assignments, to E. I. du Pont de Ne- I mours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application January 4, 1936,

Serial No. 57,635

16 Claims.

This invention relates to flexible, cellulosic pellicles and particularly to such pellicles which have been rendered resistant to change in dimensions caused by variation in humidity conditions.

Cellulosic pellicles which are obtained by coagulation or precipitation from aqueous or alkaline aqueous dispersions of cellulose or cellulose derivatives, as for example, pellicles of regenerated cellulose, glycol cellulose, cellulose glycollic acid; lowly esterified or etherified cellulose such as lowly etherified methyl and ethyl cellulose and lowly esterified cellulose acetate, are particularly useful as wrapping materials. It is essential, however, that these pellicles be flexible and further that this flexibility be maintained. Cellulosic pellicles of this character seem to depend on the presence of moisture to impart flexibility and the softeners which have been used heretofore have been selected because of their hygroscopic nature. It has been considered necessary to have the softener assist, by virtue of its hygroscopicity, in the retention of a suitable amount of moisture although the softener has possessed, also, a certain degree of lubricating or plasticizing action so :3 that the pellicle does not become too brittle when practically all of the moisture has been removed. In the absence of a softener other than water, these pellioles become so brittle when dried as to be substantially useless as wrapping tissues. Iii) In the presence of the hygroscopic softeners of the prior art, the loss of moisture is retarded and generally speaking, the flexibility is maintained by the moisture present.

Regenerated cellulose pelllcles (for convenience the invention will be described in terms of this species) are quite sensitive to changes in moisture content, not only as regards flexibility, but also as regards dimensions. Increase in moisture content causes a swelling of the cellulosic 40 structure, while decrease in moisture content causes a shrinking apparently due in part to collapse of the cellulose micelles and in part to the loss of water molecules from between the V micelles. I

Regenerated cellulose wrapping tissues are subject, therefore, to two major defects which develop simultaneously and which are primarily dependent on the softener, namely embrlttlement and deformation. Previous attempts to control 50 or prevent embrittlement, however, have not been (Cl. ill-$8) successful in controlling the deformation for, as mentioned above, the softeners selected have been highly hygroscopic and have really resulted in greater sensitivity to moisture conditions, thereby increasing the expansion and contraction 5 of wrapping tissues or similar pellicular structures.

Thus it is that great care is required in wrapping boxes in regenerated cellulose pellieles to provide for these conditions. For example, if a 10 cereal box is wrapped in regenerated cellulose sheeting (prepared in according with the prior commercial methods and softened with glycerin which is the usual hygroscopic softener) and set aside for storage, it may be subjected to a variety 5 of humidity conditions before it reaches its ultimate consumer. During this time, if the humidity is high, the regenerated cellulose may expand until the wrapper becomes loose around the box and in some cases even baggy and wrinkled. On 20 the other hand, if the humidity is low, loss of moisture from the regenerated cellulose will cause the wrapper. to contract and this may cause buckling .of the box walls, or if the box is sumciently rigid, the Wrapper itself may burst. Thus, under such a variety of humidity conditions,. packages wrapped in regenerated cellulose sheeting may have an unsightly and undesirable appearance as the result of the deformation of the regenerated cellulose. Obviously, this. defeats the very important purposes of regenerated cellulose wrappers which are intended to protect and beautify articles wrapped therein.

To overcome the troublesome deformation, wrapping machinery has been designed to allow for a certain slack or looseness in the wrap. This obviously will eliminate the effects of contraction, but cannot help the expansion enacts; indeed, it makes them worse. The use of moistureprooied regenerated cellulose pellicles does not eliminate the trouble because the application of a moistureproofing coating retards but does not prevent the deformation, which latter is in the ultimate not appreciably affected. These practical means do nothing more than attempt to make the best of the situation and make use of regenerated cellulose sheeting as it is available. No attempt is made to change the inherent properties of the cellulosic material and so" a series of drying rolls.

provide a wrapping which will have improved properties and characteristics.

It has been found in the manufacture of regenerated cellulose pellicles that the degree of deformation is, at least in part, due to the orien: tation of the cellulosic miscelles. Thus, if a sheet of regenerated cellulose is made in such a way as to provide substantially uniform tension in all directions, the degree of deformation will be substantially the same in all directions. Practically, however, regenerated cellulose pellicles are made in a continuous process wherein the pellicle, after its formation, is drawn through a series of treating and purifying baths and finally over It is necessary, therefore, in order to advance the pellicle through the cycle of treatment, to apply a certain amount of tension on the pellicle in the direction of travel, usually referred to as the machine direction. This results in an appreciable difference in the degree of deformation if measured in the machine direction, as compared to the degree of deformation if measured at right angles to the machine direction, i. e., the transverse direction. However, in any given process the conditions are such that the degrees of deformation in the machine and transverse directions usually bear a sufficiently constant relationship so that for test purposes it is usually sufficient to determine the degree of deformation in one direction only, usually the machine direction.

For the purposes of thisv specification the degree of deformation, which may be called simply deformation, is the per cent change in length of a cellulosic pellicle as measured in the machine direction in accordance with the followingprocedure: Strips of material are allowed to come to equilibrium with an atmosphere of substantially relative humidity at a temperature of 35? C. and their length accurately measured. The strips are then allowed to reach equilibrium in an atmosphere of substantially 0% relative humidity at the same temperature and their length again accurately measured. The difierence in length divided by the length as originally measured multiplied by gives the per cent deformation over the given humidity rangeat 35 C. and figures so obtained are conveniently called the deformation. Thus, a sample strip having an original length .of 10 inches and a contraction in length of 0.42 inches would be said to have a deformation of 4.2.

It is the object of this invention to provide means for reducing the deformation of cellulosic pellicles. It is also an object of the invention to effect simultaneous softening and reduction of deformation. It is a further object to provide a method whereby the aforementioned objects may be accomplished in an economically feasible manner and without entailing essential modification of apparatus customarily used in. the manufacture of such pellicles.

More specifically, it is the object of the invention to provide means for the production of cellulosic pellicles which will show less deformation than the cellulosic pellicles presently commercially available.

Specifically, it is the object of the invention to produce a pellicle of regenerated cellulose suit-. able for use as a wrapping tissue which will show a degree 'of deformation of not in excess of 3.0 when tested in accordance with the method described above. This object also includes the production of a regenerated cellulose pellicle which is combined with a softening material and which shows a deformation not in excess of 3.0.

The above and other objects of the invention may be accomplished by impregnating into the cellulosic pellicle, preferably while the latter is in a wet or gel state, an appreciably watersoluble, high-boiling (i.e. relatively non-volatile at ordinary temperatures and pressures), limitedly hygroscopic material which is stable, essentially colorless and odorless, preferably nontoxic and which is preferably also a softening agent for the cellulosic material. the reduction in deformation which may be obtained depends on the particular impregnant used and to a certain extent on the amount employed.

The term iimitedly hygroscopic is intended to include substances which will absorb 1-80% of their weight of water when exposed alone in a thin layer to an atmosphere of substantially 95% relative humidity at a temperature of 25 C. over a period of hours. The test for hygroscopicity is carried out as follows: A small sample of about 0.5-2.0 grams of thoroughly dried material is spread evenly over the bottom of a weighing bottle (conveniently about 2% in diameter and 11 5" deep) and the weight of the sample accurately measured. The open weighing bottle is then placed in a chamber in Whichan atmosphere of substantially 95% relative humidity and a temperature of 25 C. is maintained. The humid The extent of atmosphere may be maintained conveniently by means of a sulfuric acid solution (9 parts of water to 1 part sulfuric acid) contained within the percentage increase in weight based on the -ori-. ginal weight of the sample represents the hygroscopicity of the material. Thus, if 1 gram of a substance absorbs 0.25 grams of water under the conditions described, it will be said to have a hygroscopicity of 25.

The test for hygroscopicity as outlined above is satisfactory for determining whether or not a given substance is suitable for the purposes of the invention. There may be other tests of equal utility for determining hygroscopicity which would give different numerical values. Obviously, such methods should be calibrated against the method outlined if they are chosen for use. Based on the method described, those substances suitable for use in the practice of the invention will have a hygroscopicity of 1 to 80.

Generally speaking, the test of water-solubility is applied first to determine the suitability of substances for use in the practice of the invention. If the substance is at least 1% and not more than 40% soluble in water at 20 C. its utility is indicated. In the case where the solubility is more than 40%, the hygroscopicity should be determined and in the event that this value should be above 80, the substance will be useless for the purposes of the invention.

The third physical criterion relates to volatility. The substances must be high-boiling, thatis, substantially non-volatile at ordinary temperatures and pressures, and should be preferably though not necessarily liquids. Obviously, if substances of appreciable volatility were to be used, they would eventually disappear from the cellulosic pellicles rendering them thereby brittle, fragile and unsuitable for use as wrapping tissues or other applications. It has been found that substances of appropriate water-solubility and hygroscopicity which have a boiling point of 135 C. or higher; at a pressure of 12 mm. of mercury and preferably of 150 C. or higher (at 12 mm.) will be suitable for use.

In the preferred form of the invention substances having limited hygroscopicity and appropriate boiling-point and which are at least 4% soluble in water at C. will be used to impregnate the cellulosic pellicles to effect a reduction in deformation. Substances of this character may be introduced into the pellicles by means of a simple water solution. In the case where it is desired to use a substance which is less than 4% soluble, it may be convenient to increase the concentration by adding a watermiscible organic solvent such as methanol, ethanol, acetone or the like although in no case should more than by volunie of such solvent in the water-solvent mixture be necessary; Substances which require .more organic solvent are unsuited for the purposes of. the invention. Inasmuch as the use of a portion of organic solvent entails additional expense, it is desirable to use those substances which are at least 4% soluble although the less soluble substances will find many useful applications. I

Although the principal objective of the invention relates to the reduction of deformation, it is a part of the invention to provide also a flexible pellicle. This can be done frequently byproperchoice of the impregnating agent so that deforming softener".

structure.

it will be at one and the same time a softener For simplicity, such an agent may be termed a nonties may be combined with these in certain agents, which will have particular advantage.

Among the non-deforming softeners which are especially suitable for use there are a number of ether-esters derived from ether acids. These may be synthesized by the esterification of. monoor polyhydric alcohols with carboxylic acids which contain ether groups in their molecular As the alcohol portion of the ester, mono-, di, or polyhydric alcohols, preferably aliphatic, although alcohols containing cyclic or aromatic groups are not excluded, which may optionally contain one or more ether groups, may

be used. Preferably primary alcohols are used.

although secondary or even tertiary alcohols may be employed to advantage. As, illustrative of the alcohols which are useful, ethylene glycol, di-

ethylene glycol, ethoxyethanol, methoxyethanol, glycerol, or the like, may be mentioned. The

acid portion of the ester may be provided by ether-acids such as methoxyacetic, ethoxyacetic, alpha methoxypropionic, diglycollic or dilact ylic acids.

Although the preparation of the esters does 'not constitute a part of the present. invention, v since they may be prepared in any of the usual ways, it should be understood that those esters useful to the invention are. simple, non-resinous organic ether-acid esters. Accordingly, any combination which might lead to a resinous material should be avoided unless care is taken to prevent the formation of a resinous material.

Furthermore, it is understood that only those ether-esters of the type described which satisfy the requirements for hygroscopicity, water-solubllity and non-volatility will be useful.

As illustrative of specific materials which are useful in the practice of the invention, a number of compounds are listed in the following table together with data as to water-solubility and deformation, the latter figures being obtained when regenerated cellulose pellicles were impregnated as will be described in more detail below.

Obviously, other proper-- The compounds listed in table are representative and it is to be noted that simple'organic chemical compounds are effective for the purposes of the invention. All of the substances listed satisfy the criteria for operability previously discussed.

The impregnating agents of the invention may be introduced into cellulos c pellicles in the same way that glycerol is usually introduced in the manufacture of present-day commercial regenerated cellulose sheeting. Thus, a sheet of gel regenerated cellulose which has been purified and washed is impregnated with an aqueous glycerolsolution, the excess of such solution removed by suitable means and the sheet dried. I l'he commercial operation is continuous and the time of immersion in the glycerol bath varies with the present inwith a bath which may contain conveniently about 4-6% of the non-deforming agent. After removal of excess bath, as by squeeze rolls and the removal of excess water by drying, the final product will contain a suitable amount of non-deforming agent. Obviously, the amount of such agent in the final-product will be adjusted to suit the desired properties expected in the final sheet so that the bath concentration may be more or less than 4% as occasion demands. It has been found however that the effectiveness of most non-deforming agents appears .to approach a limiting value so that excessive amounts do not produce ally a bath concentration of about 4% will be found satisfactory fora realization of good nonaqueous solution of diethylene glycol diethoxy acetate. The resultant sheet, after drying shows a deformation of about 2.1, whereas a similar sheet impregnated with glycerol shows a deformation of 4.0-4.4. Both sheets contain ap-' final product is of the I sufficient improvement to justify their use. Usuv proximately 14% of the impregnating agent. In this case the product is flexible, durable, transparent and shows a reduction in deformation as compared with the glycerin softened sheet amounting to approximately 50%.

In the above table a number of compounds has been indicated, each of which is capable of use in the production of a regenerated cellulose pellicle which will show a deformation of not more than 3.0. It is to be understood that the compounds mentioned are illustrative. The list given is by no means exhaustive and this disclosure is intended to embrace all organic compounds'or mixtures of the class defined which have properties as previously set forth. These compounds may be used alone or in combination with each other.

The hygroscopicity of the compound chosen for use will determine, in large measure, the amount of reduction in deformation which will be observed. Thus, compounds having a hygroscopicity of from 60 to 80 will show a deformation of the order of 2.7 to 3.0, while if the hygroscopicity is from 25 to 60, a deformation of the order of 2.3 to 3.0 will usually be obtained tion of a non-deforming agent or non-deformall ing softener in accordance with the principles set forth herein.

The cellulosic pellicles obtainable by means of the present invention are particularly suited to wrapping purposes since the reduced deformation substantially eliminates warping, swelling, wrinkling and breakage. Similarly, the lamination of such pellicles to materials such as paper or fabric is facilitated since there is less tendency'for the laminated product to curl, buckle or wrinkle. When stacks of sheets are stored, there is a lesser tendency for contiguous sheets to stick-a common experience with glycerolsoftened sheets. Generally, the durability, particularly at low temperatures of C. or less, is improved. The process described is especially advantageous from an economic viewpoint inasmuch as it offers a method of producing this new type of cellulosic pellicle without alteration of present manufacturing equipment. While it is preferred that the cellulosic pellicle be impregnated with a solution of the nondeforming agent while the pellicle is in the gel state, i. e., before it is dried and while it still contains a. large amount of water, it is within the broad scope of the inventionto impregnate a film which has been dried and then rewetted. Although the invention has been described with specific reference-to films of regenerated cellulose prepared by the viscose process since it is-in this field that the invention is of greatest utility, it will be understood that thebroad scope of the invention includes the treatment of other ypeset cellulose film cast from aqueous or aqueous alkaline solutions. Thus, the non-deforming agents referred to above may be used with advantage in the treatment of films produced from cuprammonium cellulose solutions,

, from aqueous alkaline solutions of. glycol cellulose' or cellulose glycollic acid, from aqueous alkaline solutions or. lowly etherified cellulose, .e. g. lowly etherified methyl cellulose or lowly etherisolutions of lowly esterified cellulose. e. g. lowly esterified cellulose acetate.

Since the invention is capable of considerable variation and modification, any change from the above specific details and examples which conforms to the spirit of the invention is intended to be included within the scopeof the claims.

I claim:

1. Water-sensitive film suitable for use as a wrapping tissue formed from an aqueous alkaline cellulosic solution and containing, as a deformation-restricting agent, a softening agent for said film comprising an ester of a carboxylic acid containing an ether group having a hygroscopicity of from 1 to 80, a boiling point of at least 135 C. at a pressure of 12 mm. of'mercury, and a solubility of at least 1% in water at 20 (2., said ester being present in suflicient quantity to restrict the deformation of the film to a. maximum of 3.0.

2. Water-sensitive film as defined in claim 1 characterized in that said ester is diethylene glycol diethoxyacetate.

3. Water-sensitive film as defined in claim 1 characterized in that said ester is diethylene glycol dimethoxyacetate.

4. Water-sensitive film as defined in claim 1 characterized in that said ester is bis-ethoxyethyl diglycollate.

5. Regenerated cellulose film suitable for use as a wrapping tissue containing, as a deformation-restricting agent, a softening agent for said film comprising an ester of a 'carboxylic acid containing an ether group having a hygroscopicity of from 1 to 80. a boiling point of at least 135 C. at a pressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 0., said ester being present in sufficient quantity to restrict the deformation of the film to a maximum of 3.0.

6. Regenerated cellulose film as defined in claim 5 characterized in that said ester is diethylene glycol diethoxyacetate.

7. Regenerated cellulose film as defined in claim 5 characterized in that said ester is di ethylene glycol dimethoxyacetate. Y

8. Regenerated cellulose film as defined in claim 5 characterized in that said ester is bisethoxyethyl diglycollate.

9. A process for reducing the deformation of restricting agent being a softener for said film and comprising an ester of a carboxylic acid containing an ether group having a hygroscopicity of from 1 to 80, a boiling point of at least 135" C. at apressure of 12 mm. of mercury, and a solubility of at least 1% in water at 20 C.

10. A process for reducing the deformation of.

water-sensitive film as defined in claim 9 characterized in that said ester is diethylene glycol diethoxyacetate.

11. A process for reducing the deformation of water-sensltivefilm as defined in claim 9. charac terized in that said ester is diethylene glycol dimethoxyacetate.

12. A process for reducing the deformation of water-sensitive film as defined in claim 9 characterized in that said ester is bis-ethoiwethyl diglycollate,

13. A process for reducing 'the'deformation of 'fied ethyl cellulose, or .from aqueous alkaline a regenerated cellulose film which comprises impregnating said film with an aqueous solution containing a sufliclent quantity of adei'ormationrestricting agent to restrict the deformation of the film to a maximum of 3.0, deformationrestricting agent being a softener for said film and comprising an ester or a carboxylic containing an ether group having a hygroscopic; of from 1 to 80, a boiling point of at; I35 0. at a pressure of 12 mm. of mercury; a sohv hility of at least 1% in water at 20 CL 14. A process for reducing the: deformation at regenerated cellulose film as defined. in claim 13 arm characteriled in that said ester is diethylene glycol diethoxyaoetate.

15. A process tor reducing the defamation of regenerated cellulose film as defined in claim 13 characterized in that said ester is die'thylene glycol dimethoxyaoetate.

16. A process for reducing the deformation of ethyl dislycollate.

FREDERICK M. MEIGS. 

